THIS evergreen spinoff of the original New York crime series veers into almost comic-book territory tonight, with the case of a publicity-hungry serial rapist who inspires copycats and a groups of vigilantes dressed as superheroes. Fortunately, the solid cast includes the fiercest female cop on television, Detective Olivia Benson (Mariska Hargitay), who kicks some serious criminal butt and helps carry the storyline to a reasonably believable conclusion.